Future Growth of Greenhouse Gas Emissions

Without policies to mitigate (reduce or offset) the emission of greenhouse gases (GHG) the emissions of such gases are expected to continue to grow in the future.

The U.S. Climate Change Science Program (US CCSP) released its second report in 2007, entitled “Scenarios of Greenhouse Gas Emissions and Atmospheric Concentrations and Review of Integrated Scenario Development and Application.” This research produced new “reference scenarios” representing the researchers’ best estimates of future trajectories. These reference scenarios assume "business-as-usual" futures without significant policy changes. "Without significant policy changes" was defined to be "the assumption that no climate policies are imposed beyond current commitments, namely the 2008-12 first period of the Kyoto Protocol and the U.S. goal of reducing reduce GHG emissions per unit of its gross domestic product by 18% by 2012."

The researchers concluded that:

“In the reference scenarios, economic and energy growth, combined with continued fossil fuel use, lead to changes in the Earth’s radiation balance that are three to four times that already experienced since the beginning of the industrial age.”

This research also explored scenarios aimed at stabilizing the growth of GHG concentrations in the atmosphere at four increasingly stringent levels: roughly 750 parts per million (ppm), 650 ppm, 550 ppm, and 450 ppm (including multiple GHGs as CO2-equivalents)

[Note: In order to compare and aggregate different greenhouse gases, various techniques have been developed to index the
effect each greenhouse gas to that of carbon dioxide, where the effect of CO2 equals one, and their combined quantity is described as the "CO2-equivalent". In other words, the CO2-equivalent quantity would have the same effect on, say, radiative forcing of the climate, as the same quantity of CO2.]

The analysis concluded,

“The timing of GHG emissions reductions varies substantially across the four radiative forcing stabilization levels. Under the most stringent stabilization levels [450-550 ppm] emissions begin to decline immediately or within a matter of decades. Under the less stringent stabilization levels [750 ppm], CO2 emissions do not peak until late in the century or beyond, and they are 1½ to over 2½ times today’s levels in 2100.”

Further,

"In the stabilization scenarios, GHG emissions reductions require a transformation of the global energy system, including reductions in the demand for energy (relative to the reference scenarios) and changes in the mix of energy technologies and fuels. This transformation is more substantial and takes place more quickly at the more stringent stabilization levels. "

The results of the CCSP reference scenarios are similar to those of the 2000 Special Report on Emission Scenarios (SRES) of the Intergovernmental Panel on Climate Change (IPCC), although the latter explored a wider range of uncertainty in its reference projections. The SRES projected global GHG emissions, without further climate change mitigation policies, to increase by 25-90% (CO2-equivalent) from 2000 to 2030, with CO2-equivalent concentrations growing in the atmosphere to 600-1550 ppm.

Note: The first version of this article was drawn from Climate Change: Science Highlights by Jane Leggett, Congressional Research Service, February 23, 2009.

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